1. Field of the Invention
The present invention relates to a surface emitting laser device with a monolithically integrated monitor photodetector and a method for fabricating the same, and more particularly to a surface emitting laser device with a monolithically integrated monitor photodetector, which is combined with an automatic power control circuit, thereby being capable of more accurately controlling the surface emitting laser output power. The present invention also relates to a method for fabricating such a surface emitting laser device with a monolithically integrated monitor photodetector.
2. Description of the Related Art
As is well known, a surface emitting laser (SEL) has been highlighted as light emitting devices. Such a surface emitting laser is configured to emit light in a direction along which semiconductor layers are grown. In this regard, an array of surface emitting lasers can be integrated on a single substrate. Such a surface emitting laser also has an advantage in that it is unnecessary to use an optical system for a correction of the shape of light emitted. This is because the light emitted from a surface emitting laser has a circular shape, and exhibits an intensity of a Gaussian distribution.
It is desirable for the output power of such a surface emitting laser to be maintained at a constant intensity. To this end, it is necessary to use a monitor photodetector. Where such a monitor photodetector is integrated on a surface emitting laser, there are advantages in that it is possible to reduce the manufacturing costs and to simultaneously monitor an array of surface emitting lasers.
FIG. 1 is a schematic cross-sectional view illustrating a conventional surface emitting laser device with a monolithically integrated monitor photodetector.
Referring to FIG. 1, the conventional surface emitting laser device includes a surface emitting laser (10) for emitting light in a direction, along which semiconductor layers are grown, and a monitor photodetector (60) formed on the surface emitting laser (10).
The surface emitting laser (10) includes a substrate (12), a gain medium layer (42) adapted to generate light, a pair of mirror layers, that is, a lower mirror layer (32) and an upper mirror layer (34), respectively arranged on lower and upper surfaces of the gain medium layer (42) and adapted to resonate the light generated from the gain medium layer (42), and a laser window (82) formed on the upper portion of the monitor photodetector (60) and adapted to allow the resonated light to be outwardly emitted. The surface emitting laser (10) also includes a first electrode (22) formed on the lower surface of the substrate (12), a second electrode (24) formed on the upper mirror layer (34) and provided at a central portion thereof with an opening for exposing the central portion, and an a high-resistance layer (52) sandwiched between the gain medium layer (42) and the upper mirror layer (34) and provided at a central portion thereof with an opening. The high-resistance layer (52) guides holes provided from the second electrode (24) to flow toward the gain medium layer (42) through the opening thereof.
When a forward bias is applied between the first and second electrodes (22 and 24), a recombination of electrons and holes occurs in the gain medium layer, so that light is generated. Of the generated light, only those of wavelengths meeting a resonance condition given by the lower and upper mirror layers (32 and 34) can remain. For the remaining light, the gain medium layer (42) induces those of the same wavelength and phase to be emitted, thereby eventually amplifying those light. The induced emission light, that is, laser beams, are outwardly emitted through the laser window (82).
Meanwhile, the monitor photodetector (60) includes a first doped semiconductor layer (62), an intrinsic semiconductor layer (72), a second doped semiconductor layer (64), and a third electrode (26) for outputting a signal detected by the monitor photodetector (60). The first and second doped semiconductor layers (62) and (64) have the different doping type whereas the first doped semiconductor layer (62) has the same doping type as the upper mirror layer (34).
The monitor photodetector (60) partially absorbs the light emitted from the surface emitting laser (10), thereby outputting an electrical signal proportional to the power of the absorbed light. The remaining light not absorbed by the monitor photodetector (60) is transmitted through the monitor photodetector (60), so that it is outwardly emitted through the laser window (82).
The detect signal outputted from the monitor photodetector (60) is proportional to the output power of the surface emitting laser (10) emitted through the laser window (82). Accordingly, it is possible to control the surface emitting laser (10) to output power of a constant intensity by feeding back the detect signal from the monitor photodetector (60), and controlling the drive current applied to the surface emitting laser (10), that is, the drive current applied to each of the first and second electrodes (22 and 24), in accordance with the control operation of an automatic power control circuit based on the feedback detect signal. Alternatively, the output power of the surface emitting laser (10) may be varied in accordance with a variation in drive current.
On the other hand, the surface emitting laser (10) emits not only the induced emission light, but also spontaneous emission light. This spontaneous emission light is different from the induced emission light in terms of characteristics in that it consists of mixed light of different wavelengths and different phases. For this reason, the light detected by the monitor photodetector (60) includes both the induced emission light and the spontaneous emission light. Accordingly, the detect signal of the monitor photodetector (60) is influenced by the spontaneous emission light. As a result, it is difficult to accurately control the power of light emitted from the surface emitting laser (10), based on the detect signal outputted from the monitor photodetector (60), because of the spontaneous emission light.
Therefore, a first object of the invention is to provide a surface emitting laser device with a monolithically integrated monitor photodetector, which is capable of effectively cutting off photocurrent resulting from spontaneous emission light while allowing only photocurrent, resulting from laser light, to flow through the monitor photodetector, thereby achieving an accurate control for the power of light emitted from a surface emitting laser thereof.
A second object of the invention is to provide a method for fabricating a surface emitting laser device with a monolithically integrated monitor photodetector, which is capable of accomplishing the first object.
In accordance with one aspect, the present invention provides a surface emitting laser device comprising:
a surface emitting laser including a substrate, a lower mirror layer, a gain medium layer, and an upper mirror layer sequentially grown on an upper surface of the substrate, a first electrode formed at a lower surface of the substrate, and a second electrode formed on the upper mirror layer and provided at a central portion thereof with an opening for exposing the central portion, the surface emitting laser serving to emit light in a growth direction of the layers in response to a drive current applied to the first and second electrodes; and
a monolithically integrated monitor photodetector formed on a portion of the upper mirror layer exposed through the opening of the second electrode, the monitor photodetector serving to partially absorb the light emitted from the surface emitting laser, thereby outputting a detect signal for the light,
wherein the monitor photodetector comprises
a first doped semiconductor layer, an intrinsic semiconductor layer, and a second doped semiconductor layer sequentially grown on the portion of the upper mirror layer exposed through the second electrode,
a third electrode formed on the second doped semiconductor layer and provided at a central portion thereof with an opening for exposing the central portion of the third electrode, and
lower and upper insulating layers respectively sandwiched between the first doped semiconductor layer and the intrinsic semiconductor layer and between the intrinsic semiconductor layer and the second doped semiconductor layer, each of the insulating layers having an opening at a central portion thereof and serving to remove a photocurrent resulting from a spontaneous emission light emitted from the surface emitting laser.
Preferably, each of the first and second doped semiconductor layers comprises a number of grown layers. The uppermost one of the grown layers in the first doped semiconductor layer and the lowermost one of the grown layers in the second doped semiconductor layer are made of AlxGa1-xAs (provided, 0.95xe2x89xa6xxe2x89xa61). The lower and upper insulating layers are made of oxidized AlxGa1-xAs.
More preferably, the first doped semiconductor layer comprises a Zn-doped AlyGa1-yAs layer and a Zn-doped AlxGa1-xAs layer grown in this order (provided, 0.95xe2x89xa6xxe2x89xa61; and 0xe2x89xa6yxe2x89xa60.5). The second doped semiconductor layer comprises an Si-doped AlxGa1-xAs , an Si-doped AlyGa1-yAs layer, and an Si-doped GaAs layer grown in this order. The lower and upper insulating layers are formed by laterally oxidizing the Zn-doped AlxGa1-xAs layer and the Si-doped AlxGa1-xAs layer, respectively.
The intrinsic semiconductor layer is located at a maximum internal light intensity of the surface emitting laser.
The surface emitting laser device may further comprise a high-resistance layer sandwiched between the gain medium layer and the upper mirror layer and provided at a central portion thereof with an opening, the high-resistance layer serving to guide holes to flow only through the central opening thereof. The high-resistance layer is formed by implanting protons into the upper mirror layer, or made of an oxide of AlxGa1-xAs (provided, 0.95xe2x89xa6xxe2x89xa61).
In accordance with another aspect, the present invention provides a method for fabricating a surface emitting laser device including a surface emitting laser consisting of a substrate, a lower mirror layer, a gain medium layer, and an upper mirror layer sequentially grown on an upper surface of the substrate, a first electrode formed at a lower surface of the substrate, and a second electrode formed on the upper mirror layer and provided at a central portion thereof with an opening for exposing the central portion, the surface emitting laser serving to emit light in a growth direction of the layers in response to a drive current applied to the first and second electrodes, and a monolithically integrated monitor photodetector formed on a portion of the upper mirror layer exposed through the opening of the second electrode, the monitor photodetector serving to partially absorb the light emitted from the surface emitting laser, thereby outputting a detect signal for the light, comprising, to fabricate the monitor photodetector, the steps of:
sequentially growing a Zn-doped AlyGa1-yAs layer and a Zn-doped AlxGa1-xAs layer on the upper mirror layer, thereby forming a first doped semiconductor layer on the upper mirror layer;
forming an intrinsic semiconductor layer made of GaAs on the first doped semiconductor layer;
sequentially growing a Si-doped AlxGa1-xAs layer, a Si-doped AlyGa1-yAs layer, and a Si-doped GaAs layer on the intrinsic semiconductor layer, thereby forming a second doped semiconductor layer on the intrinsic semiconductor layer; and
wet-oxidizing a structure obtained after the formation of the second doped semiconductor layer to laterally oxidize the Zn-doped AlxGa1-xAs layer and the Si-doped AlxGa1-xAs layer, starting from side surfaces thereof, while preventing central portions thereof from being oxidized, thereby forming lower and upper insulating layers, respectively (provided, 0.95xe2x89xa6xxe2x89xa61; and 0xe2x89xa6yxe2x89xa60.5).
In the surface emitting laser device according to the present invention, photocurrent resulting from a large part of spontaneous emission light emitted from the surface emitting laser is removed by nonradiative recombination centers located at respective interfaces between the lower insulating layer and the intrinsic semiconductor layer and between the intrinsic semiconductor layer and the upper insulating layer. Accordingly, it is possible to accurately control the power of light emitted from the surface emitting laser device through the laser window by feeding back a detect signal outputted from the monitor photodetector to the surface emitting layer, as drive current, via an automatic power control circuit.